This invention relates to a novel structure and method for eliminating blocked apertures caused by charged particles on a shadow mask of a color television picture tube and more particularly to a structure and method for salvaging color picture tubes in which charged particles attached to the beam intercepting interior surface of the shadow mask are rendered conductive so as not to deflect the transmitting portions of the electron beams from the proper apertures in the shadow mask.
During the manufacturing and handling of a color television picture tube, both conductive and nonconductive particles may be trapped or generated within the tube. Typical rejection rates due to such particles average about 1/2 of 1 percent for new tubes and as high as 5 to 10 percent for reworked tubes. Conductive particles include carbonized fibers, soot, aluminum flakes and weld splash. Nonconductive or insulative particles usually comprise glass and phosphor. Glass particles may be introduced into the tube during the reworking of tubes when the tubes are renecked, or the glass particles may be generated inside both new or reworked tubes, for example, from cracked stem fillets, or mechanical damage from the friction of the bulb spacer snubbers against the glass during gun insertion. Glass particles can also be generated by crazing of the neck glass and the glass support beads during high voltage processing or from electron bombardment of the glass.
Conductive particles cause picture imperfections asuch as dark spots on the screen if the particles physically block the apertures in the shadow mask. The spots or shadows from conductive particles blocking the shadow mask apertures will appear on the screen to be approximately the same size as the particles in the mask apertures.
On the other hand, insulative particles which are charged negatively by the electron beams will cause deflection of the beams by coulomb repulsion. Therefore, these particles can cause picture imperfections such as screen spots when attached to the mask without physically blocking the mask apertures. Furthermore, it has been observed that the insulative particles, in addition to causing screen spots, also cause color misregister of the electron beams. The color misregister creates a "halo" effect resulting from the electron beams being deflected and striking the phosphor elements surrounding the obscured region.
An apparatus for removing charged particles from a conductive element, such as a shadow mask, of a color picture tube is described in U.S. Pat. No. 3,712,699 issued on Jan. 23, 1973 to Syster. The apparatus requires that the vacuum integrity of the tube be interrupted by removing the neck portion of the tube. As pointed out herein, the renecking or rework operation is a major cause of particle scrap so the apparatus disclosed in the Syster patent is only a partial solution to the problem. Furthermore, after the cleaning and rebuilding procedure disclosed in the Syster patent, the tube must be reprocessed. During reprocessing (exhaust, spot knocking, high voltage aging, etc.), additional particles may be generated.
Thus a structure and procedure is required by which the vacuum integrity of the tube is maintained, but the most troublesome particles, i.e., the nonconductive charged particles affixed to the beam intercepting interior surface of the shadow mask are "eliminated" by rendering them conductive.